214 PLANT LIFE 



place to glance at a few examples, in order to 

 gain some knowledge of the general character 

 of the sexual process itself so far as we at 

 present understand it. At the same time, we 

 shall be in a better position to appreciate the 

 bearings of its elaboration on the evolution 

 of the series of higher plants. 



If we once more take as our starting-point 

 a relatively simple unicellular plant such as 

 Chlamydomonas, we find that under certain 

 conditions it continues to grow and to multiply 

 itself vegetatively (see p. 15). After a time, 

 however, and under certain altered nutritive 

 conditions, sexual reproduction sets in (Fig. 

 24). The young individuals which have been 

 recently liberated from parent cells, after 

 swimming about for a while, undergo a change. 

 The living protoplasmic body slips out of 

 the cellulose skin, and swims as a naked cell 

 in the water. Very soon these cells are 

 observed to approach one another in pairs, 

 Two individuals become attached, and then 

 gradually coalesce. The cilia disappear, and 

 the now motionless zygote becomes spherical 

 and surrounds itself with a new cell wall. 

 Chemical changes continue to go on within 

 its body, for the chlorophyll loses its green 

 colour and gives way to a red pigment. 

 Later on, and after a longer or shorter period 

 of rest, the green colour returns, the cell 

 reawakens to vegetative activity, its contents 

 divide, and new chlamydomonas individuals 

 are produced. 



